Correct, because it's built to be completely recoverable. Not because
it's an aircraft -- that fact has no special virtue -- but because the
designer really cared about complete recoverability, and was willing to
make compromises elsewhere to achieve it.

Quote:

If you only had a delta V of
6km/s instead of 9 or so the craft could be made considerable simpler...

Yeah, but unfortunately, 9 or so is what you need to get to LEO. You can
do that with two stages without too much trouble, or one stage with some
difficulty. (Expendable stages with SSTO-class performance were built in
quantity in the early 60s.) This has nothing to do with energy efficiency.

If you're using two stages, whether you try to make the lower one look
like an aircraft doesn't matter too much, except that it's much more
expensive that way. Building a big Mach 5 rocket is a whole lot easier
than building a big Mach 5 aircraft. (To date, *nobody* has built an
aircraft that could get to Mach 5 entirely under its own power. The first
Mach 5 rocket flew in 1942.)

Aiming for higher energy efficiency makes the problem *worse*, not better,
because systems with higher energy efficiency use more mass, and these
systems mostly are mass-limited, not energy-limited. They have lots of
energy; obsessing over energy efficiency is just plain dumb. It's mass
that's hard to come by, either because it takes bigger tanks or because
you need big, heavy, costly machinery to gather it from the air. (Hint:
making tanks bigger is much easier than building efficient hypersonic
air intakes.)
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | henry@spsystems.net

Only with considerable difficulty. The poor T/W and limited speed range
of airbreathers hurts them badly for acceleration missions.

What about a turbo fan where the fan and duct ass. pops off at a
certain speed?

What about it? Ever built one? Ever seen one? As far as I know,
nobody's ever done it. In certain ways it would be quite a trick.
And a well-optimized turbofan core doesn't resemble a Mach 2 turbojet
all that closely.

The best Mach 2 turbojets have thrust/weight ratio of about 10:1, and it's
been a long hard struggle to get them that far. For the last several
decades, that number has incremented at maybe 1 per decade, despite a lot
of effort. And they don't work very well at Mach 3, never mind Mach 6;
nor do they work very well beyond perhaps 50,000ft. The T/W ratios of jet
engines which do are not nearly as good.

The T/W ratio of the best LOX/kerosene rocket engines passed 100:1 in the
1960s. They work just fine from Mach 0 to Mach 25 and beyond, and they
work *better* at 300,000ft than at sea level.

Quote:

The propulsion equivalent of a gear box.

A propulsion system that needs to shift gears to go from Mach 0 to Mach 2
is rather badly in need of rethinking, not to say scrapping. Especially
since it will need to shift gears again -- somehow -- to go much faster.

Quote:

Rocket fuel and rocket engineering are cheap and easy.

That why I don't believe N. Korea has a nuke yet.
Kim Il just shoots off rockets to get attention.

If he'd put the money and resources into rocket engineering that he's
put into nuclear-weapons engineering, he'd have beaten China to manned
orbital flights.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | henry@spsystems.net

Yeah, but unfortunately, 9 or so is what you need to get to LEO. You can
do that with two stages without too much trouble, or one stage with some
difficulty. (Expendable stages with SSTO-class performance were built in
quantity in the early 60s.) This has nothing to do with energy efficiency.

Or three stages. Then you can make the first stage out of a low cost

Mach 0.8 plane

Quote:

If you're using two stages, whether you try to make the lower one look
like an aircraft doesn't matter too much, except that it's much more
expensive that way. Building a big Mach 5 rocket is a whole lot easier
than building a big Mach 5 aircraft.

A Mach 0.8 aircraft can be off the shelf. Or even better, if you
beleive scaled composites, composite technology allows the building of
low cost custom large aircraft, as proposed for T-space.

This doesn't help much with delta V, but improves launch location
(altitude and position). It also forces the designers in to a lauch
with minimal immediate pre-launch preperation.

ianparker2@gmail.com wrote:
If you look at the Shuttle you will find that quite a lot of delta V is
at relatively low speeds >1km/s. The Shuttle rises vertically and
turns. Chucking mass out at 4km/s for vertical acceleration and turns
is quite clealy inefficient. We want to "push" at 1km/s or less. To do
this you need an airbreather accelerating a large mass. OK a rocket may
be the best way of getting from 2-8km/s but it is certainly not the
best way of getting to 2km/s from the ground.

Right on! When I try to explain power transfer ,
I use a Harrier jet compared to a helicopter and

Power = Force x Velocity .

Force = Power / Velocity .

The Force is what you want to lift, and the Velocity
is the speed of the air/exhaust you're using to do it.
Obviously a Harrier needs a lot more Power than a
helicopter and thus has a higher fuel consumption.
Is that what you guys understand too?

Who cares? What we're trying to optimize is cost, not dry mass of the
launch vehicle or any sort of efficiency you can measure on the launch
vehicle.

As Henry said, fuel and oxidizer are cheap, at least they are if you pick
reasonable propellants. A favorite of mine is LOX/kerosene. LOX is one of
the cheapest fluids on the planet since it's made from air! Even with
today's "high" oil prices, kerosene isn't that expensive, especially
compared to today's launch costs. Also, kerosene is far more dense than
something like LH2, so it's a good choice for a first stage fuel.
Furthermore, if you cool your engines with the LOX instead of the kerosene,
like some Russian engines do, you don't even have to be very picky about the
refinement of the kerosene since you don't have to worry about kerosene
coking and clogging up the cooling passages of the engine. This not only
lowers fuel costs, but should also make it easier to reuse the engine.

Also, tankage is cheap and the thrust to weight ratio of a rocket engine is
better than that of an airbreather built to operate over a wide range of
speeds (subsonic to several Mach numbers). That and a rocket engine isn't
as complex as such an airbreather. Plus you have the fact that there simply
aren't that many air breathing engines to choose from if you expect them to
operate over a wide range of speeds. Most air breathing engines are
optimized for subsonic cruise, with a few optimized for supersonic cruise
(e.g. SR-71, Concorde, and some modern jet fighters).

So exactly why would anyone prefer a costly, complex, heavy air breathing
engine for rapid acceleration of a launch vehicle, when you can use a more
simple, lighter, cheaper, LOX/kerosene first stage engine instead?

What about a turbo fan where the fan and duct ass. pops off at a
certain speed?

That would be bruttally complex. Better to just jettison the whole engine
and switch to a turbojet, if that's what you really want to do. Engine
weight is a relatively small percentage of overall weight for most
airframes.

Quote:

The propulsion equivalent of a gear box.

Lots of aircraft have gearboxes in the drivetrain...nothing new there.

What I don't understand is why they were never able to control the
Harrier and other vertical takeoff planes better. It seems that's one
problem where computer controls easily could handle any instability.

The Harrier is a really old design...computer control of unstable aircraft
has come a long way since it was first developed.

Quote:

It should be as safe as any plane.

I don't think we'll ever get that far...a VTOL aircraft is, almost by
necessity, dynamically unstable. Failure of the control system will kill
it. Not true for most aircraft.

ianparker2@gmail.com wrote:
It is and it isn't. If what you are saying is that the fuel cost of the
Shuttle is small then you are right. If what you are saying is that
energy efficiency is not important you are wrong.

No, we are right. We are so far from propellant costs dominating total
costs that energy efficiency is a second-order issue right now, and will
be for a long time.

Energy efficient is important because it affects the ratio of payload
to deadweight. Not cost of propellant.

Rand Simberg wrote:
ianparker2@gmail.com wrote:
It is and it isn't. If what you are saying is that the fuel cost of the
Shuttle is small then you are right. If what you are saying is that
energy efficiency is not important you are wrong.

No, we are right. We are so far from propellant costs dominating total
costs that energy efficiency is a second-order issue right now, and will
be for a long time.

Energy efficient is important because it affects the ratio of payload
to deadweight. Not cost of propellant.

The "ratio of payload to deadweight" is not a relevant measure of
goodness. Cost of delivering the payload is. Energy efficiency is
important only if it reduces costs. Likewise for raw performance, "high
tech" solutions, and cosmetic features.

Retract to where? The blades are quite a bit longer than the hub diameter,
and it's not like there's a lot of unused space inside a jet engine.

Quote:

It would be nice to have the assembly counter rotate down to the ocean
like a maple tree seed. Fans are already made of carbon.

Depends on whose engine you're looking at. Most fan's aren't carbon.

Quote:

And a well-optimized turbofan core doesn't resemble a Mach 2 turbojet
all that closely.

Instead of expanding in the last turbine the gas is reheated in the
after burner.

You can't seriously be discussing propulsion efficiency and then throw an
afterburner into the equation. Afterburner is just a way of augmenting
thrust with a *ferocious* efficiency penalty...you do it because you have
to, not because you want to. A supercruise engine would do far better from
an efficiency standpoint.

Quote:

High speed propulsion is going to require some sophistication no matter
what you try.

Highest speed propulsion = rocket = lowest sophistication.

A missile with a cheap solid rocket motor can outrun the best jets ever
built.

ianparker2@gmail.com wrote:
If you look at the Shuttle you will find that quite a lot of delta V is
at relatively low speeds >1km/s. The Shuttle rises vertically and
turns. Chucking mass out at 4km/s for vertical acceleration and turns
is quite clealy inefficient. We want to "push" at 1km/s or less. To do
this you need an airbreather accelerating a large mass. OK a rocket may
be the best way of getting from 2-8km/s but it is certainly not the
best way of getting to 2km/s from the ground.

Right on! When I try to explain power transfer ,
I use a Harrier jet compared to a helicopter and

Power = Force x Velocity .

Force = Power / Velocity .

The Force is what you want to lift, and the Velocity
is the speed of the air/exhaust you're using to do it.
Obviously a Harrier needs a lot more Power than a
helicopter and thus has a higher fuel consumption.
Is that what you guys understand too?

Who cares? What we're trying to optimize is cost, not dry mass of the
launch vehicle or any sort of efficiency you can measure on the launch
vehicle.

As Henry said, fuel and oxidizer are cheap, at least they are if you pick
reasonable propellants. A favorite of mine is LOX/kerosene. LOX is one of
the cheapest fluids on the planet since it's made from air! Even with
today's "high" oil prices, kerosene isn't that expensive, especially
compared to today's launch costs. Also, kerosene is far more dense than
something like LH2, so it's a good choice for a first stage fuel.
Furthermore, if you cool your engines with the LOX instead of the kerosene,
like some Russian engines do, you don't even have to be very picky about the
refinement of the kerosene since you don't have to worry about kerosene
coking and clogging up the cooling passages of the engine. This not only
lowers fuel costs, but should also make it easier to reuse the engine.

Also, tankage is cheap and the thrust to weight ratio of a rocket engine is
better than that of an airbreather built to operate over a wide range of
speeds (subsonic to several Mach numbers). That and a rocket engine isn't
as complex as such an airbreather. Plus you have the fact that there simply
aren't that many air breathing engines to choose from if you expect them to
operate over a wide range of speeds. Most air breathing engines are
optimized for subsonic cruise, with a few optimized for supersonic cruise
(e.g. SR-71, Concorde, and some modern jet fighters).

So exactly why would anyone prefer a costly, complex, heavy air breathing
engine for rapid acceleration of a launch vehicle, when you can use a more
simple, lighter, cheaper, LOX/kerosene first stage engine instead?

Your post is data-rich, and Henrys opinion is respected.
I'm preaching to the choir but I figure our bench-marks
are the Saturn V and/or the Shuttle.
It's obviously cost-effective to use an engine that costs
10x more if you can use it 50 times...ok, that was the
rationale of the Shuttle over the SatV, and it worked.

An air breathing re-usable Mach 10+ ramjet for a 2nd
stage is reasonable quest, the 1st gets her up to
Mach 1 using cheapo SRB's, and the 3rd does
orbital insertion, and if you want that can be re-entered
and reused, that's all proven.
I suggest we Build & Blast until we get it right,
then we own cheap LEO, that's the goal!
Regards
Ken S. Tucker
PS: Is the ISP of a ramjet >2000